Statistical methodology to classify and prioritize superseded current transformers replacements

摘要

Iron core current transformers (CT) rely on magnetic flux over ferromagnetic material to deliver a reliable copy of the primary current to protection relays. Under asymmetric fault condition and if it is not properly sized, the core of a CT can saturate, and do not deliver loyal current on its secondary windings to the protection relays. That situation can lead differential protection schemes of power system elements, such as busbars, transformers and lines, to trip when they shouldn't. Once a CT saturates, there is no reliable way to predict the behavior of a differential relay facing distorted currents due to saturation. Today's modern digital protection relays act much faster than the older electromechanical relays, and saturation happening on the first cycle of a fault or of a fault reclosing can lead to undue trip. Utilities and regulatory agencies realize now that while protection relays have been modernized, replaced for new digital ones, hundreds or thousands of CTs, due to their long life span around 30 years, remained on site, leading to risks of undue differential trips. All of these CTs need to be replaced for newer ones with equivalent core cross section much times larger or new technologies. This work aims to propose a methodology to classify and prioritize these replacements. A small case study is shown, demonstrating the benefits of the use of the methodology.